Current thin film solar cells on flexible substrates are limited to amorphous silicon on a thin metal foil (usually stainless steel) and copper indium gallium diselenide (CIGS) on metallic or polyimide foils. Currently, thin film cadmium telluride (CdTe) solar cells are produced only on glass, but work is being done on a flexible substrate embodiment. To be useful in a solar power system, all types of solar cells must be electrically interconnected serially with other similar solar cells to raise the voltage levels and minimize I2R losses that would otherwise occur due to high currents. Cells deposited on large rigid sheets of glass generally use a system of scribes applied between different process steps and at specific locations to interconnect the cells over the entire sheet. This procedure is called “monolithic integration”. Such a method is difficult to implement on flexible substrates because of the accuracy required for both the placement and depth of the scribes. Additionally, flexible substrates enable roll-to-roll processing which can become less desirable economically if the process is interrupted to implement the scribing operations, even if they could be readily accomplished. It is less common for thin film solar cells to be deposited on rigid glass or metallic wafers similar to silicon wafers.
Conventional crystalline or polycrystalline silicon solar cells are formed on individual wafers, which then must be interconnected. Current collecting grids and buss bars are usually formed by screen-printing a pattern with silver bearing inks that are subsequently cured at high temperatures (on the order of about 700° C.). Silicon cells have an antireflection coating made from silicon nitride, which is transparent but non-conducting. During the curing stage the silver penetrates the silicon nitride coating and forms an ohmic contact to the silicon cell. The usual grid pattern consists of a series of fine straight and parallel lines spaced a few millimeters apart with two or three wider lines (buss bars) running perpendicular to the pattern of fine lines. The resulting structure provides a surface on the buss bars to which interconnecting “Z” tabs can be attached by conventional soldering or bonding methods. The cell current is collected by the relatively narrow grids and transmitted to the relatively wider buss bars, which then become the connection points to the next cell. The “Z” tab structure provides a flexible interconnection between cells that helps reduce damage due to thermal expansion and contraction during use. An advantage of this method over monolithic integration is that the cells can be tested and sorted for performance prior to module build. In this way the module performance is not limited by the lowest performing cell in the string.
The screen-printing method applied to thin film flexible solar cells has met with only limited success. There are at least two problems associated with this method. First, the thin film cells cannot survive the high temperatures needed to adequately cure the silver inks. As a result of lower curing temperatures, some of the ink carriers and solvents remain in the grid line structure causing the metallic particles not to be well fused together. Both of these effects lower the conductivity of the grid lines and buss bars, and limit the solderability to the printed buss bars. Alternatively, the interconnection may be made with conductive epoxies, but it is generally inferior to soldering. Second, since the surface finish of useful flexible substrates is normally much rougher than that of glass or silicon wafers, many more defects exist which can become shunt sites if conductive ink is allowed to flow into them. This problem can be somewhat mitigated by first printing a much less conductive material, like a carbon ink to initially fill any defects, and then over printing with the silver ink. Consistently good results are difficult to achieve, since anything short of perfect printing registration causes extra shading loss as well as increased potential shunting. In addition, the cost of the materials and equipment is relatively high.
U.S. Pat. No. 5,474,621, which is entirely incorporated herein by reference, proposes using metallic wires as grids, but with the wires coated with carbon fibers of sufficient length to avoid or lessen the chance of being forced into defects. In this method, the wires are attached to the top electrode (transparent conductive oxide, or TCO) of the thin film amorphous silicon solar cells during the process of laminating them into modules. In effect, the prior art approach of first printing a carbon-based ink pattern is replaced with carbon fibers that have much less chance of causing shunts in film/substrate defects, and at the same time provide a fusing type of protection against sustained heavy shunt currents. The wire size and spacing must be selected so as to carry the current generated by the cell without generating significant resistive losses.
U.S. Pat. Nos. 4,260,429 and 4,283,591, which are entirely incorporated herein by reference, teach methods for coating conductive wires with a polymer that contains conducting particles. A limitation of these methods is that problems with defect-induced shunts can still exist because of smaller conductive particles in the distribution. Improvements to these methods were taught in U.S. Pat. No. 6,472,594, which is entirely incorporated herein by reference.
More recently, U.S. Patent Application Publication No. 2010/0043863 to Wudu et al. (“Wudu”), which is entirely incorporated herein by reference, teaches a solution where a trace (or wire) pattern is formed first on a transparent carrier and then applied to the solar cell. Various teachings of Wudu are shown in FIGS. 1a, 1b and 1c. In FIG. 1a, a wire 520a is applied to a carrier 550 in a serpentine pattern. The loops in the wire at the ends of the pattern remain just on the carrier on one edge, but extend substantially past the carrier on the opposite edge. When applied to the cell, the carrier covers the active region of the cell while the extended loops provide an area to make electrical contact to the next cell. A detailed cross-sectional view of the carrier and the wire of Wudu are shown in FIG. 1b. Carrier 550 consists of two materials, a transparent polymer sheet 550a (for example, a thin PET sheet) and a thermal setting adhesive 550b. Wire 520 is partially embedded in the adhesive and consists of a normal wire 520A (for example, copper) that is coated with a protective and lower conductivity material 520B, like nickel. FIG. 1c shows the carrier and wire of Wudu after it is laminated to the top transparent conductive layer 510b of solar cell 510. The deformation of the polymer sheet creates a force that holds the wire in electrical contact to the top conductive layer of the solar cell 510. When this structure is laminated to a glass top sheet (not shown), an additional layer of adhesive (for example, EVA) is required. The adhesive must be thick enough to fill around the wire geometry.
While the above-described construction of Wudu represents an improvement to the art, especially in eliminating inks and the problem of conductive material getting into defects, it nevertheless has three features that remain undesirable. For instance, round wire 520 of Wudu makes contact with the cell only along the small area represented by the tangent line 1 in shown in FIG. 1c. This creates practical difficulties in being able to consistently keep the contact resistance low in a manufacturing environment. Another problem is that the thickness of thermal adhesive 550b must be controlled rather precisely. If too thin, it might not completely fill the space around the wire; if too thick, it might underflow the round wire with sufficient pressure to lift the wire and break its electrical contact to the cell. Still another problem with Wudu is that the two-layer carrier represents extra material that leads of increased manufacturing costs.
There is thus a need for improved interconnect systems and methods.